Plastics foam is now commonly produced by in situ methods whereby the gas or frothing agent is generated during the polymerisation process. The chemical components in liquid form are mixed together, generally by mechanical means, and then whilst still in the liquid stage and prior to any appreciable reaction taking place are injected into the mould or cavity to be filled. Foaming then takes place within the mould or cavity and involves volumetric change up to 30 fold according to the density required and the degree of under- or over-packing induced by design.
In situ foaming has a number of disadvantages, the principal ones being that the foam so produced is anisotropic in its strength properties, high internal mould pressures are often created and perfect filling, especially in complex cavities, is difficult to achieve.
Pre-froth foam on the other hand, i.e. foam which is first produced and then injected or placed as foam in the mould or cavity, should in theory be much more isotropic in nature, generate much less pressure and, because of its free-flow characteristics, make perfect filling of complex cavities much easier.
Pre-frothing is discussed in the article "Frothing" by R. E. Knox (`Chemical Engineering Progress`, Vol. 57, No. 10, Oct. 1961). It has hitherto been difficult with pre-frothing to control quality and density and to obtain reproducibility.
The principle of pre-frothing is to use a low boiling point liquid, normally a fluorocarbon (refrigerant) as the gas-producing agent. Unlike the in situ foam process, where gas production is in proportion to the rate of polymerisation, the fluorocarbon flashes off into a gas almost instantaneously following a rapid pressure drop.
This semi-explosive nature of its change of state from liquid to gas tends to shear the viscous plastic cell walls giving rise to the phenomena known as gas break-out. Having once broken out from the boundaries of a viscous mass the gas cannot satisfactorily be induced back into it. Gas break-out makes metering the correct quantity of fluorocarbon meaningless and therefore control of density, quality and reproducibility becomes impossible.
The invention has for its object to provide method and apparatus for pre-frothing which avoids gas break-out, and which enables control of density and quality.